# What is Motion: Definition + 999 Threads

In physics, motion is the phenomenon in which an object changes its position over time. Motion is mathematically described in terms of displacement, distance, velocity, acceleration, speed, and time. The motion of a body is observed by attaching a frame of reference to an observer and measuring the change in position of the body relative to that frame with change in time. The branch of physics describing the motion of objects without reference to its cause is kinematics; the branch studying forces and their effect on motion is dynamics.
If an object is not changing relatively to a given frame of reference, the object is said to be at rest, motionless, immobile, stationary, or to have a constant or time-invariant position with reference to its surroundings. As there is no absolute frame of reference, absolute motion cannot be determined. Thus, everything in the universe can be considered to be in motion.Motion applies to various physical systems: to objects, bodies, matter particles, matter fields, radiation, radiation fields, radiation particles, curvature, and space-time. One can also speak of motion of images, shapes, and boundaries. So, the term motion, in general, signifies a continuous change in the positions or configuration of a physical system in space. For example, one can talk about the motion of a wave or about the motion of a quantum particle, where the configuration consists of probabilities of occupying specific positions.
The main quantity that measures the motion of a body is momentum. An object's momentum increases with the object's mass and with its velocity. The total momentum of all objects in an isolated system (one not affected by external forces) does not change with time, as described by the law of conservation of momentum. An object's motion, and thus its momentum, cannot change unless a force acts on the body.

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1. ### Absolute motion analysis of pulley-spring system

Hello! I have this problem from an old exam I'm trying to solve. The problem is in Swedish so I've translated it: NOTE that I accidentaly wrote $$C\neq 1$$ in the picture below. The correct problem statement is above. But that part is not what I have problems with. The answer key says "if the...
2. ### Vertical circular motion

So far I believe that speed changes during vertical circular motion, and its very hard to get uniform circular motion that is in the vertical plane. This is because there is a difference in vertical height between the bottom/top of the circle so at the top the object must have done work against...
3. ### B Generating Energy from Molecular Motion

This question really troubles me. Since we all know that the molecules are in perpetual random motion, why can't we make a perpetual motion machine by using molecular's always-moving energy?? I‘m a Grade 9 student and I'm unsure about this. :doh::doh::doh:
4. ### Engineering Determining how far a body will move up an inclined plane and the time to reach the highest point

15cos25 - 0.25N = ma.
5. ### A Equations of Motion for Torsional Inverted Pendulum on a moving cart

Hello I am trying to figure out the equations of motion for the following scenario: a mass m is held upright on by a massless rod mounted on a cart that is being pulled by a linear force. The caveat is that the rod is mounted on the cart via a torsional spring which causes a restorative torque...
6. ### Projectile Motion Problem Involving Initial Velocity and Gravity Only

Ok, so to start, the easy part, the velocity in the horizontal x direction (vx): vx = 20m/(2.5s) = 8m/s So far so good, but then for the vertical y velocity (vy), my answer is different than the book. I use the equation for position in the y direction, set the y value to 0 (the 2.5s it takes...
7. ### Circular motion doubt: Angular velocity vector for general planar motion about a point in the plane

I thought the opposite should be true since its a general planar motion its not necessary that the magnitude of radius vector is constant so the change in direction and magnitude of radius vector should be generated by the perpendicular velocity vector and parallel velocity vector respectively ...
8. ### Find the initial velocity ##U##

Now in determining the initial velocity; in my understanding, if ##s=1.8## then we consider the stone's motion from the top to the ground. Why not consider ##s=3.6##, the total distance traveled by stone from start point ##t=0##? Is it possible to model equations from this point? The stone...
9. ### B Terminology for motion in the solar system, ecliptic maybe?

Just looking for terminology here. If an object moves through the solar system, what is the simplest terminology that relates to how the object moves through the solar system? What are the words for if an object moves towards the sun (radially inwards) away from the sun (radially outwards)...
10. ### I How Does a Polygon Topple Without Sliding or External Force?

How does a polygon shaped body's toppling shifts from one edge to next adjacent edge when the object is already in motion without any sliding and no external force is applied ? Explain it with the case of hexagon or octagon both ways, with and without including CoM.
11. ### Arrow being fired to centre of target

i solved it like this... s = ut + 1/2 at^2 t= 1.08 (from part a) u= 65 sin4.30 a= 9.81? or -9.81 the answer said -9.81 why? wouldn't acceleration change from -9.81 to +9.81 because it moves up then down??? its soo confusing...
12. ### Rolling Motion (Stone Gets Stuck in a Tire Going Forward)

So I thought the stone would initially experience acceleration in the backward (leftward) direction then continually accelerate in the inward direction of the tire (i.e. upward then rightward then downward then leftward, etc.) as the tire moves forward. But the answer is immediately upward...
13. ### Where Did I Go Wrong in Proving Harmonic Motion in an Adiabatic Process?

TL;DR Summary: Problem said that the ball moves in a harmonic motion and asked to prove it. The process is adiabatic Problem said that the ball moves in a harmonic motion and asked to prove it. The process is adiabatic. I did the development, but at certain point I'm having a problem. The...
14. ### Sign of Work (Negative or Positive)

Im confused I did 850*9.8*14.7 = 1.2x10^5 Why is it not negative since the force is opposite to the motion?
15. ### Physics 1 Parabolic Motion Question Confusion

Problem: A small forest animal jumps with an initial speed of v0 = 15.0m/s and travels to a maximum height of 2.160m. What horizontal distance would the animal travel if the launch angle is i) 45.0 degrees or ii) 42.0 degrees? Correct Answer: i) 24.95m ii) 25.02m My professor solved this by...
16. ### Two dimensional motion problem (kicking a soccer ball into the goal)

I did this too fast idk if I'm wrong So for the x component use the formula d = rt / delta x = v*t 26 = vcos(theta) y component use the displacement formula 19.6 = vsin(theta) tan^-1(19.6/26) = 36.5 degrees the answer key says 20.2 degrees idk whats wrong
17. ### B Is perpetual motion a forbidden topic in the scientific community?

I've only just clocked this to be the case. I asked chat gpt and the say big bang has a lot of merit apparently

idk
19. ### Motion of Sphere Rolling Down Rotating Cone

I am trying to understand the motion of the sphere in the image above, and I am a bit confused about the motion. How does the ball move down the cone? Will the rotation of the cone cause the ball to rotate with it, and which direction would the static friction be in? What does the path the ball...
20. ### Need a real life example where a partial derivative is used in motion

my group is preferring the ue of partial derivative to find the acceleration of a car or the projectile motion of something being launched
21. ### Circular motion of a rollercoaster car on a loop-the-loop

Can someone please check if I got the correct answer. Thank you! I got: Fy= N + Fg = mac N + mg = mv^2/r g = v^2/r r = v^2/g r = (48.61)^2/9.8 r =...
22. ### B Launch of a rocket - its initial displacement and velocity

so then when we model the velocity of the motion of a rocket (e.g., in a mathematics report), is it reasonable to assume that the initial velocity equals zero? I also apologise for my lack of information if I made a huge mistake in my question 🙏🏻
23. ### Projectile motion only provided acceleration diatance

TL;DR Summary: Find initial vertically upward speed of the ball Find horizontal speed of the speed Find angle How to: Find initial vertically upward speed of the ball Find horizontal speed of the speed Find anglei try to solve it but it didn't work
24. ### Projectile motion when only given distance and acceleration

TL;DR Summary: Find horizontal velocity? I have no idea how to solve the problem, the question only provide distance 16cm(h),3.6cm(v) and acceleration = 0
25. ### Finding value of g given a motion map

I tried using all equations of motion but couldn't get the correct answer. Any hint would greatly help. I've tried doing this for two days now!
26. ### Adjustable Hand Stroboscope, how to use?

What kind of lab activity would you use an Adjustable Hand Stroboscope for? https://prolabscientific.com/Education-supplies/Physics/Force-and-motion/Stroboscopes/Hand-Stroboscope-Adjustable.html Would it purely be a qualitative activity? I can't imagine you can do anything really quantitative...
27. ### In a constant acceleration motion, Is it possible to have constant speed?

I think it's not possible. In 3D for constant acceleration we have : ##\vec v = \vec v_0 + \vec a t## It's a line in 3 dimension so velocity's magnitude(speed) is changing with time. I appreciate any better idea.
28. ### Help with question on motion: Avoiding a rear-end collision

The question : A car and a truck are both traveling with a constant speed of 20 m/s. The car is 10 m behind the truck. The truck driver suddenly applies his brakes, causing the truck to slow to a stop at the constant rate of 2 m/s2. Two seconds later, the driver of the car applies their brakes...
29. ### I Observation of the effect of gravity on the motion of antimatter

The Alpha Experiment at CERN has finally produced a paper on whether antimatter falls towards the earth under gravity. The research confirms that antimatter acts identically to regular matter in regards to gravity. Observation of the effect of gravity on the motion of antimatter. Anderson...
30. ### How many discs will slide off the board as it decelerates?

I don't undertand the equation. It is Newtons's second law of motion, so it decribes a force that acts on a single disc relative to the ground. So when the force is proportional to velocity, shouldn't it be ##-bv##? Because the dics's velocity is ##v## relative to the ground. Relative to the...
31. ### I Newton's Second Law - variable mass case

Dear Experts. In a problem where we need to calculate the acceleration of a satellite in a force free space which sweeps and collects interplanetary dust and a certain rate (dm/dt), I believe that the equation F = m(dv/dt) + v(dm/dt) can be used, by putting F=0 and substituting the function...
32. ### Solve Disk & Coil Spring Motion: Get Amplitude Help

I was able to solve part 1 but I am not not getting how to find new amplitude of the motion . Please help me
33. ### Analyse motion of an oscillator: x(t)=0.2cos(12*pi*t)

Hello. I have tried to solve it using x-t Graph. We know that period of this function is ##T=\frac {1}{6}s##. Then I've used ##x(t)=0## to find the times in which the oscillator is at ##x=0##: ##t=\frac {k}{12} + \frac {1}{24}## for ## k \in Z.## Now I can draw x-t graph. We should check time...
34. ### Translating reciprocating motion into a 45 degree rotation

I'm trying to design a mechanism to translate reciprocating motion into a 45 degree rotation. Here's the idea: A pin will push against the part marked in red, causing part of the desired rotation. Then, when the pin is pulled back, its interaction with the blue part will complete a 45 degree...

36. ### Solve First Year Harmonic Motion Problem: Cylinder Rolling

Thank you guys for taking the time to read this - I'm decently struggling with first year and need some tips on how to properly conceptualize problems and learn what the right approach is on certain problems. Have a wonderful day, again thank you for checking this post out!
37. ### Total Force on a Point Charge in Motion

As the observer is moving, there will be a magnetic force. Electric Field of the Rod = λ/2πεr r̂ Electric Force on the Point Charge = qλ/2πεr r̂ Magnetic Force on the Point Charge = q(vxB) = qvB n̂ = qv(µI/2πr) n̂ = qv(µλv/2πr) n̂ = µqλv²/2πr n̂ Total Force = Electric Force + Magnetic Force
38. ### A Are equations of motion invariant under gauge transformations?

We know that all actions are invariant under their gauge transformations. Are the equations of motion also invariant under the gauge transformations? If yes, can you show a mathematical proof (instead of just saying in words)?
39. ### I Rolling Motion direction of Velocity center of mass

Hi I have come across something confusing in rolling motion. If an object moves with a positive V_cm meaning to the right its angular velocity will be clockwise or negative. The formula is V_cm=wR but for a positive V_cm you get a negative w as it moves clockwise if V_cm is to the right...
40. ### Majorana Fermions: Lagrangean and equations of motion

$$i \gamma^{\mu} \partial_{\mu} \psi = m \psi_c \\ i \gamma^{\mu} \partial_{\mu} \psi_c = m \psi$$ Where ##\psi_c = C \gamma^0 \psi^*## Show that the above equations can be obtained from the followong lagrangian  L = \overline{\psi} i \gamma^{\mu} \partial_{\mu} \psi - \frac{1}{2} m \left...
41. ### Tusi discovering laws of planetary motion before Kepler?

I was reading about the Tusi couple and read it "as a solution for the latitudinal motion of the inferior planets, and later used extensively as a substitute for the equant". Since the Tusi couple is related to plotting out an ellipse, did Nasir al-Din al-Tusi already discover the laws for...
42. ### Circular Motion: A coin on a rotating disk

I believe I've solved this problem, however, I got through it pretty quickly and since it's the last problem on the assignment, I feel that I may have had an oversight. For part a, I got: fs=md(α^2)(t^2) and for part b, I got: ω=Sqrt((µs*g)/d) Could someone confirm my answers? I've attached a...

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44. ### Circular Motion - Newton's Second Law: Bead on a Rotating Hoop

For whatever reason, I'm having a hard time conceptualizing this problem. I understand that the tangential components of all forces involved need to cancel out in order for the bead to be stationary. I also understand that there is a mgsinθ in the negative θ-hat direction. What I don't...
45. ### B The synchronization of clocks and the relativity of motion

In his thought experiment on the relativity of simultaneity, Einstein synchronizes two clocks A and B (at either end of a rod) to the clock C in a stationary frame, while A and B (the rod) is moving relative to C. The equations Einstein finishes with demonstrate how observers moving with A and...
46. ### I Can angular motion exist outside of a gravity field?

I can't find the answer anywhere here's my question. can a centrifuge exist outside a field of gravity. More specifically, in a theoretical void of nothing without stars in view or any point of reference for comparison how could motion like spinning or acceleration exist?
47. ### I Is it possible, that one physics law is "Everything is in motion?"

Is it possible, that one physics law is "Everything is in motion?" (since zero doesnt belong to nature)...if so, does that implies that black holes move?
48. ### I On whether the motion of a Foucault pendulum bob is comparable to ballistics

A recurring question is: while the motion of a polar Foucault pendulum is fairly straightforward, the case of a non-polar Foucault pendulum is quite difficult to visualize. In 2020, on physics stackexchange someone submitted that question and I contributed an answer. In a comment to another...
49. ### QM Eigenstates and the Notion of Motion

I'll risk a quick off-topic answer here, since I think it's straightforward QM, not vague "interpretation" stuff. :oldbiggrin: In QM (e.g., Ballentine p81), for a free particle, ##H = \frac12 \, M\, V\cdot V + E_0##. So in the ground state ##|E_0\rangle## we have ##H|E_0\rangle = E_0...
50. ### How to prove that motion is periodic but not simple harmonic?

TL;DR Summary: Prove that a sum of trigonometric ratios is periodic but not not simple harmonic. We need to prove that ##x = sin{\omega t} + sin{2\omega t} + sin{4\omega t}## where ##x## is the displacement from the equilibrium position at time ##t##. I can see that each term is a SHM, but...